mohaji: You're making progress! However, Leaf Blade + Wish, Leaf Blade + Yawn, and Leaf Blade + Baton Pass aren't there when they should be. 3rd-gen-only event moves don't play nice with 4th-gen only egg moves. Also, do you plan to go back to Tentacool/Tentacruel, or would you like me to handle that one?

Added new Counter (HG/SS egg move) combinations. Also changed some other things: Sweet Scent + Water Gun has always been legal through an event Poliwag with Sweet Scent, and BubbleBeam + 3rd gen Ludicolo-only move tutor was missed last time.

I decided to take this one for myself because there were enough issues with the original list that a full redo was necessary. AncientPower illegal combos were neglected in the Platinum list (it seems Cubone and Marowak can't learn AncientPower via tutor). This includes AncientPower + Perish Song + 3rd-gen-only tutor, because Lapras can supply Cubone with the two egg moves only in the 4th generation. Additionally, the previously-listed illegal combination Belly Drum + Iron Head + Screech is legal by starting with a Munchlax (which gets Screech via level up), evolve it into a Snorlax, Heart Scale Belly Drum onto it, and tutor Iron Head onto it.

No changes in the HGSS transition, but most of the Focus Energy + Stomp combinations were missing. Focus Energy + Stomp was illegal in the 3rd gen (the necessary parent is the 4th-gen Cranidos), and I can't find any other way to get Counter with those two moves.

No changes. This list is simple enough. The only incompatibilities come from the 4th gen ability, which is not an issue for the evolved forms because of the possibility of ability change upon evolution post-migration.

New HGSS egg moves Sludge and Yawn. HGSS Curse Corsola legalizes Curse + Mirror Coat. Missed from last time were 3rd-gen-only tutors in conjunction with two-move combinations that were illegal in the 3rd gen that became legal in the 4th gen (The only ones back in Platinum were the Uproar ones since back then Curse + Mirror Coat was illegal entirely).

With no reply from mohaji about further updating the Tentacool/Tentacruel list (a number of Muddy Water combos were still missing), I might as well post the full list I came up with for cross-checking purposes here.